As an example of an analyzer for analyzing the residual gas of a vacuum device, a quadrupole type mass spectrometer is known. In general, the quadrupole type mass spectrometer comprises an ion source, a filter section and a detection section. The ion source is provided with a filament (cathode electrode) and a grid (anode electrode), and when filament current is supplied to the filament, the filament is heated, and thermal electrons are emitted toward the grid.
The filter section has four rod-like electrodes (quadrupole electrodes) arranged between the ion source and the detection section. The configuration of the four rod-like electrodes is such that they are arranged lattice-like, symmetrical to and parallel with each other, and are wired such that opposing rod-like electrodes have the same electrical potential. A voltage (+U+V cos ωt and −U−V cos ωt) in which DC voltages U with the same amplitude but opposite positive and negative polarities, and AC voltages V cos ωt whose phases are 180 degrees apart, are superimposed, is applied to the two pairs of rod-like electrodes.
The detection section uses a secondary electron multiplier, or a Faraday cup, for example, in order to detect the ion current.
In the case where a partial pressure is measured, a filament current is supplied to the filament to emit thermal electrons. The thermal electrons emitted from the filament collide with the gaseous molecules of the gas to be measured, and the gaseous molecules are ionized. Furthermore, the thermal electrons are scavenged by the grid, becoming an emission current, and flow between the filament and the grid. When supplying the filament current, the filament current is supplied while being controlled such that the emission current becomes constant.
Among the ionized gaseous molecules, only ions that have a mass-to-charge ratio corresponding to the amplitude V of the AC voltage oscillate stably, pass through the quadrupole electrode, and reach the ion detection section. The other ions diverge midway, and either collide with the quadrupole electrodes or are guided to the space outside the quadrupole electrodes. Ion current is detected as an output from the ions reaching the ion detection section.
In such a quadrupole type analyzer, when a partial pressure is not measured, it is theoretically not necessary to supply a filament current to the filament. However, if current is supplied to the filament in a state in which current has not been supplied, the filament is heated and emitted gas is generated. Moreover, if the filament is heated, the grid in the vicinity of the filament is heated by the radiant heat thereof so that emitted gas is also generated from the vicinity of the filament. Because the emitted gas has an effect on the measurement results of the partial pressure, it is necessary to wait until emitted gas stops being generated in order to measure the partial pressure accurately. Therefore, heretofore, even when a partial pressure is not measured, a filament current has been supplied to the filament (for example, refer to the following Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-33075